Chemical and/or biological field-effect transistor sensors (FET sensors) having nanometer dimensions typically need to be exposed to an analyte solution (e.g. an electrolyte) so as to enable sensing. In order to protect the sensor FET's gate dielectric during processing, it is often covered by an insulating material. A cavity typically must, therefore, be etched at the end of the manufacturing process to expose the gate dielectric of the FET and allow exposure to the electrolyte. This is a delicate step to perform as making such a cavity at the end of the process typically requires etching down several hundred nanometers or even micrometers deep while stopping on the gate dielectric, which is typically only a few nanometers thick. Additionally, achieving the desired overlay accuracy of the deep etching with respect to the gate dielectric at the end of the manufacturing process is a non-trivial challenge. It is very difficult to meet these requirements with good reproducibility with current manufacturing techniques.
One way to address some of these issues is disclosed in US 20140073039 A1. A method is disclosed comprising forming a plurality of FETs on a semiconductor substrate, wherein a subset of the FETs is for forming biological FETs (BioFETs); forming a poly-silicon sacrificial plug over a portion of the gate electrode for the BioFETs; forming contacts on the FETs; forming a multi-layer interconnect (MLI) over the FETs; etching the MLI to expose at least a portion of the sacrificial plug, thereby forming a portion of a micro well; and removing the sacrificial plug (and optionally the gate electrode) using a non-plasma etch, thereby forming a second well portion of the micro well. The non-plasma etch uses gaseous xenon fluoride (XeF2) as an etchant, or the non-plasma etch is a wet etch based on potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), or a hydrofluoric acid/nitric acid/acetic acid mixture (HNA) as etchants. After removal of the sacrificial plug, a partially fabricated BioFET is obtained. Further steps may involve replacement of the gate dielectric in the BioFETs (typically SiO2) by a biologically compatible high k dielectric (e.g. Si3N4, Al2O3, HfO2, TiO2 or Ta2O5). Additional steps may also include coating a sidewall and/or a bottom of the microwell with a first and/or second coating, respectively.
However, several issues remain with this approach. For example, because the BioFETs in US 20140073039 A1 are typically still only partially fabricated, the delicate sensor parts (e.g., gate dielectric) are still prone to be damaged during the multiple process steps that follow the removal of the sacrificial plug. Furthermore, the proposed non-plasma etches (e.g., TMAH based) will typically not only etch poly-silicon, but also other materials such as Al, Al2O3, Ti, and TixOy. However, it is common for FET sensors to comprise exposed surfaces of these materials, such as Al bond pads and/or Al2O3 passivation and anti-fouling layers. The disclosed method thus generally requires the sacrificial plug to be removed before exposing these materials, yet further increasing the number of processing steps that may still damage the delicate sensor parts (e.g., gate dielectric).